[NS A33] Unify changing the default network

makeDefault() will be called in rematchAllNetworksAndRequests
in all cases and its first job is to set mDefaultNetwork.

The old code checks if the currently processing request is
the default request and assigns mDefaultNetwork if it is,
doing it earlier than in the new code iff the new default is
not null. However there is no good reason to assign this
member earlier in the non-null case than in the null case,
it's simpler if the same code path is used in both cases.
mDefaultNetwork is also not used between the old place where
it was set and the new place where it is set, so changing
the timing of the assignment has no observable side effects.

Test: ConnectivityServiceTest
Change-Id: Id47c19b73650ba66bff73b07edb8fd95c707e699